Instead, get ahold of your buyer's board application and review it to make sure that (1) the application was properly submitted and (2) there was nothing overlooked and there were no glaring errors, e.g. a missing zero somewhere, or failure to include all of the assets, says lawyer Sandor Krauss.

At a minimum, try to get some feedback to avoid a future turndown. While the board has no obligation to tell you why they rejected your buyer, don’t be afraid to ask.

“You should very carefully and very nicely have your attorney make a lovely, sweet phone call saying, ‘I’m not second-guessing you, but could you please tell me what the problem is so we don’t waste anymore of your time,'” suggests closing lawyer Jerry Feeney. “The way to frame it is that you're trying not waste anyone’s time.' More than half the time I’ll get some insight saying it that way."

2. Check the contract -- you may have grounds for keeping the deposit. "Make sure that the buyer did not breach the contract. Make sure all applications--mortgage and co-op--were timely under the contract and that the buyer complied with each and every contractual provision," says Krauss.

3. Revisit the aesthetics. “If the apartment wasn’t decluttered, staged or updated the first time, this is the time,” says Mindy Feldman of Halstead.

Even if it was, “Perhaps extra fluffing of pillows and fresh flowers or a slightly altered furniture arrangement could freshen up the apartment,” says Stribling’s Furniss.

4. Update your listing. Take seasonal photos so the listing doesn’t look like it’s been sitting on the market for months, suggests Feldman. She also recommends creating a new floorplan and updating the copy of the listing, too.

5. Consider a price change (and maybe even raise it) Keeping your asking price within $5,000 of the originally listed price, "shows you believe your home has value, and that there will be another buyer to show interest," says Frances Katzen of Prudential Douglas Elliman.

That said, you and your broker should look at the market carefully to see if it’s gone up or down since you accepted the price on the deal that just died in order to decide how to make a price change, says Stribling’s Stimpson.

Check out how the big listing sites like StreetEasy and The New York Times break up their pricing and change yours enough so that the apartment falls into a new category, attracting a brand-new set of eyes. For example, if your place was priced at just over $1 million, consider lowering it to just-under $1 million.

Sometimes even a price increase will help sell an apartment.

Amelia Gewirtz of Halstead once took over a listing for a Fifth Avenue co-op that simply couldn’t sell. The problem, she says, was that the co-op board insisted on 50 percent cash down, and the apartment price was too low -- under $1 million -- to attract buyers with that kind of cash on hand.

“People looking for apartments under $1 million often can’t afford to put 50 percent down," says Gewirtz. "Sometimes you want to take a moment to step back, think who the appropriate buyer is for the apartment, and ask yourself, ‘Am I missing my audience’?”

6. Get the word out... “All real estate agents keep sign-in sheets at their open houses. Go back to those lists and let those buyers (and their agents!) know your apartment is back on the market. Focus on moving forward in doing all you can to find a new buyer,” says Brad Malow of Charles Rutenberg Realty.

7. ...and look on the bright side. “Dust yourself off, grab your bootstraps and get ready to get back to work,” saysBrian Morgan of Citi Habitats. “ As a seller, one bid should never cloud your judgment as the market is always changing ... For example, with the downtown condo market trending upward at an accelerated rate, an offer falling through could actually be a positive for sellers in that category — as they may very well receive a stronger offer in no time at all.”

8. Get more aggressive. Katzen of Prudential Douglas Elliman recommends scheduling more open houses, marketing the listing in as many publications as possible and speaking up to spread the word.

Asked if there's a risk of seeming desperate or overexposed, Katzen answers,“The ultimate goal in real estate is to reach the largest audience as possible, and marketing your home is different than negotiating. Desperation would be to continuously reduce the asking price.”

9. Promote special showing hours. Offer early in the day or evening showings once or twice a week, suggests Feldman: “In certain [usually lower] price points in particular, it’s difficult for people to leave work during the day, so it’s a good marketing tool to offer these.”

10. Create an event for the brokerage community. Get brokers and their buyers inside your property by enticing them with prizes and raffles (and not just a run-of-the-mill cocktail party), suggests Feldman.

To help sell an Upper East Side property recently, she threw a broker event with prizes like a stager consultation, a massage gift certificate and brunch at Cafe Boulud (a nearby restaurant). The incentives all related to the neighborhood somehow, and she sold the apartment soon after.

11. Get a back-up offer As you now know from experience, it's not over till you hand over the keys, so don't stop showing the apartment too soon.

“One thing I always do--and I always tell sellers who attempt to sell on their own--is to continuously show an apartment for back-up once an offer is accepted," says Malow. "Deals can fall through, especially before contract signing. The pursuit to sell your apartment should never cease merely because you have an offer on the table."

He recommends marketing the apartment and holding open houses "until you have a buyer’s signature on that dotted line. Even then, keep all open house and showing records handy just in case something goes awry," and you need to reach out to other formerly interested buyers.